Ships, submarines, and other water navigable craft are equipped with highly complex Navigation SONAR Systems that interface with the craft's central navigation system through a central navigation computer. These systems employ detailed software and/or firmware and extensive hardware on which such software and/or firmware executes. Such systems measure a ship's velocity. When measuring velocity, a SONAR transducer located on the hull of a ship transmit pulses to the ocean bottom. These pulses reflect off the ocean bottom, return to the ship, and are sensed by hydrophones located on the ship's hull. The hydrophones may be in a spatial arrangement, in which there are multiple hydrophones arranged in a spatial geometry such as a square, or in a temporal fashion, one arrangement of which consists of three hydrophones placed at three corners of a square.
When determining velocity, two or more pulses are transmitted to the ocean bottom, reflected off the ocean bottom, received back at the hydrophone array, and correlated amongst the multiple hydrophones to determine which two hydrophones best match the two pulses of interest. The velocity of a ship can then be calculated by dividing the distance between the two correlated hydrophones by twice the time differential between the receipt of the two distinct pulses by the two hydrophones.
A shortcoming of a spatial correlation sonar velocity measuring system is that it has a performance limitation for elevated ship speeds due to the fixed size of its hydrophone array. This limitation gives rise to a steadily degrading velocity error for ship speeds above a threshold value. Because of this fixed-size limitation, the hydrophone position that is best suited for the correlation may actually be outside the bounds of the hydrophone array. This out of bounds condition can occur most often as the velocity of the ship increases, such that a pulse that echoes off the ocean bottom and returns to the ship will be outside the bounds of the array because the increased speed of the ship has caused the hydrophone array to move beyond the bounds of the best correlation hydrophone position.
One manner to address this out of bounds condition is to decrease the interval between the pulses so that the likelihood of correlating pulses falling within the bounds of the spatial correlation sonar hydrophone array increases. However, a problem associated with this method is that a shorter time interval then increases the error caused by positional errors of the hydrophones in the array (i.e., installation errors) and acoustic offset errors of the hydrophones (normally caused by aging of the hydrophones). Another manner to address this problem involves increasing the physical size of the spatial hydrophone array. Yet another manner involves foregoing the use of a spatial correlation sonar hydrophone array and using temporal correlation sonar techniques.
Notwithstanding these techniques to address the problems of measuring speed at higher vessel velocities, the sonar art would benefit from a new method to calculate the velocity of a ship using a spatial correlation sonar hydrophone array. Such a technique would allow for the retention of spatial correlation sonar advantages such as excellent low ship speed performance and accurate velocity measurement of all velocity vectors.
The approaches described in this background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this background section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this background section.